This invention relates to a process for inhibiting the polymerization of vinyl aromatic compounds such as styrene during its distillation. The process involves adding to the vinyl aromatic compound a mixture of at least one nitroso compound such as N,Nxe2x80x2-di-2-butyl-N,Nxe2x80x2-dinitroso-1,4-diaminobenzene and a dinitrophenol compound such as dinitrocresol, and optionally a stabilizer compound such as an N,Nxe2x80x2-dialkyl substituted 1,4-diaminobenzene.
Styrene is one of several vinyl aromatic compounds which has considerable commercial utility. Styrene is polymerized into polystyrene which is a clear, readily colored and easily fabricated plastic with many uses. The efficiency of the polymerization process is dependent on the purity of the monomer starting material. Since the processes for producing styrene, and other vinyl aromatic compounds, will contain various reaction products including benzene, toluene, etc., the mixture is distilled to separate these undesirable contaminants. Unfortunately, the temperatures required for distillation, typically 90xc2x0 C. to about 150xc2x0 C., leads to the polymerization of the vinyl aromatic compounds. In order to minimize or prevent polymerization of the vinyl aromatic monomer, it is common to add a polymerization inhibitor to the distillation mixture.
The art discloses a variety of compounds which are claimed to inhibit polymerization. These include U.S. Pat. No. 4,050,993, which discloses the use of N,N-dinitroso-methylaniline as a polymerization inhibitor. U.S. Pat. No. 3,988,212, which disclose the use of N-nitrosodiphenyl amine in combination with dinitro-o-cresol. U.S. Pat. No. 4,013,580, which discloses the use of N-nitroso aniline derivatives. U.S. Pat. No. 4,941,800, which discloses the use of a mixture of dinitro-p-cresol and N-nitroso-diphenyl amine. U.S. Pat. No. 4,654,451, which discloses alkyl substituted p-nitroso phenols in combination with p-nitroso phenol. U.S. Pat. No. 5,034,156 which discloses N-nitrosophenyl-hydroxylamine plus hydroquinone monomethyl ether. U.S. Pat. No. 5,396,004, which discloses a phenylenediamine compound plus a hydroxyalkylhydroxyl-amine compound. U.S. Pat. No. 5,254,760, which discloses 1-oxyl-2,2,6,6-tetramethylpiperidine plus an aromatic nitro compound. U.S. Pat. No. 4,929,778, which discloses a phenylenediamine compound plus a hindered phenol compound. U.S. Pat. No. 5,312,952, which discloses the use of the reaction product of a C9-C20 alkyl phenol with sulfuric and nitric acid and, optionally, an aryl or alkyl-substituted phenylenediamine. WO9503263, which discloses 3,5-di-tert-butyl-4-hydroxy-N,N-dimethyl benzyl amine. EP-697386-A2, which discloses 4-acetylamino-2,2,6,6-tetramethyl piperidine N-oxyl in combination with 4-nitroso phenol. JP0701910-A, which discloses phosphite compounds, nitrosoamine compounds or phenol compounds. JP05310815-A, which discloses the ammonium salt of N-nitrosophenyl hydroxylamine. JP03149205-A, which discloses nitrosophenols plus dicyclohexylammonium nitrate. J01226858-A, which discloses the use of substituted nitrosobenzene. U.S. Pat. No. 4,967,027, which discloses p-nitroso phenol plus p-t-butyl catechol. J59029624-A, which discloses the use of N-nitroso compound, e.g., N-nitroso-diphenylamine and a catechol, e.g., p-t-butylcatechol.
Finally, U.S. Pat. No. 5,869,717, discloses the use of N-nitroso derivatives of unsubstituted or dialkyl substituted phenylenediamine.
Applicants have now discovered that N-nitroso derivatives of unsubstituted or dialkyl substituted phenylenediamine (diaminobenzene) can be combined with unsubstituted or alkyl substituted dinitrophenolic compounds.
As stated, this invention relates to a process for inhibiting the polymerization of a vinyl aromatic compound during its distillation. One embodiment of the process involves adding to the vinyl aromatic compound an effective amount of a mixture of: 1) a nitroso compound selected from the group consisting of 
and mixtures thereof, where R and R, are each independently hydrogen, cyclohexyl, phenyl or an alkyl group having from 1 to 18 carbon atoms and 2) a nitrophenol compound selected from the group consisting of dinitrocresol, dinitrophenol, 2-sec-butyl-4,6-dinitrophenol, and mixtures thereof.
Another embodiment of the invention is a process for inhibiting the polymerization of a vinyl aromatic compound comprising adding to the compound an effective amount of a mixture of: 1) a nitroso compound selected from the group consisting of 
and mixtures thereof, where R and R1 are each independently hydrogen, cyclohexyl, phenyl or an alkyl group having from 1 to 18 carbon atoms; 2) a nitrophenol compound selected from the group consisting of dinitrocresol, dinitrophenol, 2-sec-butyl-4,6-dinitrophenol and C1 to C18 alkyl substituted nitrophenols and mixtures of, and 3) a stabilizer selected from the group consisting of phenylenediamine, dihydroxy phenol, diphenylamine, alkyl substituted phenylenediamines, phenol, C1 to C18 alkyl substituted phenols and mixtures thereof.
These and other objects and embodiments will become more evident after a detailed description of the invention.
The present invention relates to a process for inhibiting the polymerization of a vinyl aromatic compound during its distillation. Vinyl aromatic compounds to which the instant process is applicable include styrene, alpha-methyl styrene, divinylbenzene, vinyl toluene, vinyl naphthalene and polyvinyl-benzene
In most cases this process will be practiced during distillation wherein air is not present. Anaerobic conditions are desired to stabilize vinyl aromatic compounds (inhibit polymerization). These conditions require the special inhibitors of the current invention.
The process generally involves adding a mixture of at least two inhibitor compounds to the vinyl aromatic compound during its distillation. One inhibitor is selected from the group consisting of 1) a nitroso compound selected from the group consisting of 
and mixtures thereof, where R and R1 are each independently hydrogen, cyclohexyl, phenyl or an alkyl group having from 1 to 18 carbon atoms; 2) a nitrophenol compound preferably selected from the group consisting of dinitrocresol, dinitrophenol, 2-sec-butyl-4,6-dinitrophenol and C1 to C18 alkyl substituted nitrophenols and mixtures thereof, and 3) a stabilizer selected from the group consisting of phenylenediamine (1,4-diaminobenzene), dihydroxy phenol, diphenylamine, alkyl substituted phenylenediamines, phenols, C1 to C18 alkyl substituted phenols and mixtures thereof.
In structures I and II, the R and R1 groups can each independently be hydrogen, cyclohexyl, phenyl or an alkyl group having from 1 to 18 carbon atoms. Illustrative of the alkyl groups which can be used are methyl, ethyl, propyl, butyl, pentyl, octyl, decyl, sec-butyl, iso-propyl, cyclohexane and isopentyl. Included in structures I and II above are the o, m and p isomers. Preferred compounds encompassed within structure I and II are the following compounds having structure IA and IIA. 
The amines used to obtain the nitroso compounds have structures (IV) and (V) below. 
The reaction of nitrous acid with an amine is well known in the art. A specific example of the reaction using compound IV (above) is detailed in U.S. Pat. No. 5,869,717. An outline of this method is included here for completeness. Generally, the amine is reacted with nitrous acid at a temperature of about xe2x88x9210xc2x0 C. to about 120xc2x0 C. The nitrous acid can be added or it can be generated in situ by reacting sodium nitrite with hydrochloric acid. Further, in the case of a diamine, the amount of nitrous acid added will determine whether the mono or dinitroso compounds are obtained. However, even if a 1:1 stoichiometric amount of nitrous acid:amine is added, a mixture of the mono and dinitroso compounds may still be obtained.
In some cases it is preferred to add a thermal stabilizer to the mixture of inhibitors. A thermal stabilizer can inhibit the decomposition of the nitroso compound thus resulting in an inhibitor with greater thermal stability. What this means is that the temperature range over which the inhibitor can be used is extended. Specific examples where such a stabilizer is preferred are: 1) where the temperature in a distillation tower is too hot and 2) an accidental temperature excursion. These stabilizer can be selected from the group consisting of 
and mixtures thereof where R2 is selected from the group consisting of hydrogen, phenyl, cyclohexyl and an alkyl group having from 1 to 18 carbon atoms, and R3 is selected from the group consisting of hydrogen, cyclohexyl, an alkyl group having from 1 to 18 carbon atoms and an aromatic group having from 6 to 10 carbon atoms. Preferred stabilizers are those represented by structure (IV) and an especially preferred stabilizer has structure (IVA) 
Without wishing to be bound by any one particular theory, it appears that compounds such as (IVA) inhibit thermal decomposition by the following mechanism. It is first assumed that thermal decomposition of the nitroso compounds is triggered by cleavage of the nitroso group giving nitrous oxide (as evidenced by the evolution of a brown gas upon exposure to air). The nitrous oxide then catalyzes the further decomposition of the nitrosated inhibitor compound. One possible route for this further decomposition is the nitrous oxide induced cleavage of another nitrous oxide (autocatalysis).
Nitrous oxide is known to react with any primary or secondary amines such as phenylenediamines. Thus if phenylenediamine is present when nitrous oxide is formed, then the nitrous oxide will react with the amine and not be available to induce further cleavage of the nitroso compounds. However, phenylenediamines are not the only compounds with which the nitrous oxide can react. The phenolic compounds (structures V and VI) mentioned above can also react with the nitrous oxide. The amount of stabilizer necessary to increase the decomposition temperature is at least 0.0001 wt. %, but can vary from about 0.0001 to about 1 wt. % of the mixture of nitroso inhibitors.
In order to adequately prevent polymerization of the vinyl aromatic compounds, an effective amount of the mixture of nitroso compounds and dinitrophenolic compounds must be used. Usually the total effective amount of inhibitors varies from about 10 ppm to about 2 wt. %. The relative amount of each inhibitor in the mixture can vary widely but usually for the nitroso compound it is from about 1 ppm to about 50 wt. % with the dinitrophenolic compound and A optionally stabilizer making up the remainder of the mixture.
Having added the effective amount of inhibitor plus stabilizer to the feed containing the vinyl aromatic compound, the feed is distilled at a temperature of about 65xc2x0 C. to about 150xc2x0 C. in a standard distillation column. The distillation temperature can be controlled by adjusting the pressure in the column from about 0 kPa (0 psi) to about 165 kPa (24 psi) absolute. Further, the inhibitor and stabilizer mixture may be added to the vinyl aromatic compound in any convenient manner. Usually the mixture is added in liquid form by periodically or continuously adding the required amount to the inlet distillation feed. In this respect, if the inhibitor and/or stabilizer is a solid, it is dissolved in an appropriate solvent. Illustrative of the solvents which can be used are toluene, benzene, ethylbenzene, xylene, styrene etc. Preferred solvents are ethylbenzene or the starting amine which allows for easy separation from the purified vinyl aromatic compound.